Electric capacitor



Nov. 16, 1948. G. H. FLOYD, JR

ELECTRIC CAPACITOR Filed Feb. 4, 1944y IIA Fig.

KILOVOLTS /0 MEGACYCLES ours/e cum/DER ALENGTH Ir'lve HLOTI George HlogcLJrr,

bs WW His Attorney Patented Nov. 16, 1948 ELECTRIC CAPACITOR George H. Floyd, Jr., Schenectady, Y., assigner to General Electric Company, a corporation of New York Application February 4, 1944, Serial No. 521,079

2 Claims. (Cl.` 175-41) My invention relates to electric capacitors, and to capacitors of the type which are arranged in an evacuated housing and which have application to high frequency circuits. l

Heretofore capacitors have been formed, such as those which have particular application in high frequency apparatus, which include a plurality of concentric radially spaced overlapping cylinders which are placed in an evacuated contalner, such as a tubular glass housing. Metallic end caps are sealed to the opposite ends of the housing and the tubular electrodes are secured at one end to one of the end caps, the adjacent of the electrodes being secured to opposite of the end caps. Terminals are placed on the outer surface of the end caps and the voltage impressedV across the terminals is therefore placed across the elec-` trodes. Such a capacitor is described and claimed in Patent 2,339,663, Teare, issued January 18, 1944, and which is assigned to the same assignee as this present invention.

The above described capacitor construction Ais very efficient and sturdy type capacitor but when the operative conditions, such as impressed voltage, are raised above a certain value, depending upon the size of the capacitor, the glass housing may become heated and may puncture, thus destroying the vacuum of the capacitor.

It is therefore an object of my invention to provide an improved capacitor of the above-mentioned type which is eillcient in operation and which may be run at a relatively higher operating voltage.

A further object of my invention is to provide animproved capacitor construction which will have a relatively lower ionic loss in the insulating housing which enclosed the electrodes.

Further objects and advantages of my invention will become apparent from the following description referring to the accompanying drawing, .nd the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In the drawing Fig. 1 is a. sectional side elevation of a capacitor which is formed according to my invention; Fig. 2 is a view of a portion of 'he capacitor formed according to prior construcns, and Figs. 3, 4, and 5 include curves which l be used in the description of my invention.

Referring to Fig 1 of the drawing, I have .lustrated an electric capacitor which includes a ontainer formed by a member I0 having a substantially cylindrical and somewhat convex shape with end caps II and I2 at opposite ends thereof.

The cylinder Il may be formed of any suitable material, such as a borosilicate glass or a ceramic material, and the end caps il and I2 may be formed of any suitable metallic material such as one including an alloy of iron, nickle and cobalt, and having expansion characteristics which match the particular glass or ceramic material so that the peripheries of the end caps II. and I2 may be sealed into the edges of the cylinder Ill to provide an hermetically sealed container arrangement.

Supported from the inner surface of the end caps Il and I2 are a plurality of axially centered and concentrically spaced tubular or cylindrical electrodes, I3, I3 and Il. Th'e electrodes extending outwardly toward each other from the opposite end cap are telescoplcaliy interleaved in spaced relation. In the construction illustrated in Fig. 1, two cylinders Il and Il' are shown as attached to the end cap II and one cylinder I4 to the end cap I2, but it is to be understood that any suitable number of electrodes may be provided. The length of the electrode cylinders is chosen in accord-ance with the capacity desired but in any event the outermost electrode cylinder should be approximately one-half the length of the insulating container and, as shown in the drawing, the next adjacent electrode cylinder is longer than the outermost electrode cylinder.

During the manufacture of the electrode elements each is provided with flanges Il and It for the electrodes I3 and I4, respectively. These flanges are lattached in any suitable manner to the end caps as by spot welding to suitable backing discs I'i and Il. Thus one end of the electrodes I3, I3 is attached to the backing plate I1, the inner electrodes I 3 having an outwardly positioned flange and an outer electrode I I' having an inwardly extending flange. Thus one flange may be placed on top of the other and spot welded to the backing plate I1 with a reinforcing ring I9 on top of the flanges so as to rigidly support the electrodes. The inwardly extending flange i8 is also spot welded to the backing plate Il and a reinforcing ring 20 is provided adjacent the flange Il.

One of the end caps, such as I2, is provided with an exhaust tube 2| which'ls shown as sealed oif and which communicates with the interior of the container through an opening 22 provided in the backing disc I0.

Extending outwardly from the opposite end caps I I and I2 are cylindrical members 28 and 24 which are provided with end flanges 2B and 2l, respectively, by which they are secured auch as by ateismo spot welding or bracing to the corresponding end cap. Inverted cua-shaped elements 2l and 2d fitting closely over the cylindrical members to and 24, respectively are secured thereto in any suitable marmer such as by soldering to ilorm symmetrical end terminals for the device. The cumshaped terminal .element t@ it will he olo= served encloses, the sealed-oiieir-iaust tuo-e iii thereby improving the appearance o the device.

Such a construction as described alcove is described and claimed in the alcove-mentioned Teure patent, and although it provides a very simple, convenient, and rugged capacitor cona struction, l have found that under severe opere ating conditions sueldos under high voltage cona ditions the glass container wm "eccome hot and may puncture. Thus referring to 2, l. have shown a construction the above=mentloned Teure patent which includes concentric tubular electrodes A and El. 'Under the severe operative conditions existing in the illustrated construction the glass tuce may puncture due to ionic loss heating in the vicinity oi the outer edge of the outer electrode A as' marked by the letter C. The heating is appar ently due to a high ireduency field which produces' a voltage in the glass and a subsequent or the type described in y ionic current which heats the class. The voltaire across the capacitor may loe determined ley the approximate formula i sejo (l) Assuming a constant capacitance Equation i may loe simpliiied into in f (2) In formula 3 the constant K is a constant that changes with frequency. By solving this constant K' it can loe shown that K'=K"Xire quency.- Such characteristics are shown in Fig.

4 in which degrees temperature rise is. plotted against different irequencies at various voltages.

Thus we may say T-:Kmfvi where T 'is the temperature rise above ambient temperature of the glass on the hot end of the capacitor, K" is the ionic loss constant for any one type of capacitor, i is the frequency of the radio-frequency voltage applied, and i V is the'radiofrequency voltage across the capacitor.-

The only controllable factor in this formula is the constant and I have found that this varies 'with the chemical composition of the glass and the geometry of the capacitor, ,i

l In order therefore to provide a capacitor oi the above-mentioned type-'which will have' a minimum of heating or in which the constant K will he a relatively low value, l have found that this constant may he lowered by terminating the outer electrode i3 adjacent the center ot the glass cylinder or in. the vicinity oi the center region. Further, the end d@ ci the outer cylinder is rolled over so as to provide a smooth contour and provide a minimum concentration of stress which is set un in the glass. ln order to turther decrease ythe ionic loss constant oi the capacitor the glass hulls in the center is loulged as indicated at 3i so that the end Sti will loe further from the glass loullo than is the supported end. The class hulle may loe loulged in any suitable amount and in the construction illustrated in lille. i the bulge 3i .is suilcient to cause the distance from the edse til radially to the inner suriace ci the glass to .be about twice the distance hetween the electrode it and the glass cylinder at the supported end of the electrode.

ln order to illustrate the effect of diferent lengths of the outer cylinder with a construction as is illustrated'in Fic. l l have plotted in Fic. 5 the relative values oi the ionic loss constant in per cent as a function or the outer length ci the cylinder or the per cent or full length. l't is assumed that when the outer cylinder is full length with the construction as is illustrated in Fig. 2, the constant is loll per cent. When the outer cylinder ii is halt length as illustrated in Fig. l, the dielectric loss constants is about f/s,

and when the cylinder has been shrunk so that it is no longer present as outer cylinder then the cylinder i8 loecomes the outer cylinder and the dielectric loss constant is equal to 25 per cent of that when the cylinder iii is full length. It will beseen that when the outer cylinder i3' is shrunk to zero the glass at the supported end is now twice the distance from the end of the inner cylinder i3 than it would he from the outer cyl-` inder i3' and as heating in the glass varies lnversely as the square oi distance trom the source of the 4radio-frequency field, this latter case will be 1/4 of the value as when the outer cylinder is fun length.

It will be seen from a close inspection of Fig. 5 that the minimum loss is not exactly at 50 per cent of full length lout at slightly less than that as it appears that the concentration of the eld isslightly in advance of the free end but it may be said that the minimum constant is when the outer cylinder terminates at approximately the center and when the ioluge center is used for the glass'.l In case the straight glass container were used in the type shown in Fig. 2 the con stant K!" would be one-half for a half length cylinder than for a full length cylinder and for the outer cylinder shrunk to zero the constant would be one-fourth that of the full length cylinder. v

Although I have shown and described particular embodiments of my invention, I do not desire to be limited to the particular embodiments described, and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of my invention.

What I'claim as new and desire to vsecure by Letters Patent of the United States is:

1. An electric capacitor including a pair of concentrically .arranged overlapping cylindrical electrodes. anenvelope for said capacitor including a cylinder of insulating material and conductive means sealed to opposite ends of said with said cylindrical electrodes, said' electrodes being of substantially diii'erent axial lengths with the longer of said electrodes arranged within the shorter electrode, said shorter electrode terminating substantially midway between the ends of said cylindrical insulator.

2. An electric capacitor including a pair of concentrically arranged overlapping cylindrical electrodes, an envelope for said capacitor including a cylinder of insulating material having a central bulbous portion of greater diameter than the ends thereof and conductive means sealed to opposite ends of said cylindrical member and connected respectively with said cylindrical electrodes, said electrodes being oi' substantially different axial lengths with' the longer oi.' said electrodes arranged within the shorter electrode. said shorter electrode terminating substantially 4midway between the ends oi said cylindrical in- The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,510,341 Proctor Sept. 30, 1924 1,870,141 Regerbis Aug. 2, 1932 1,956,368 Wilde Apr. 24, 1934 1,983,371 Hillebrand Dec. 4, 1934 2,119,070 Bouwers May 31, 1938 2,339,883 Teare Jan. 18,1944 

